Loudspeaker system



July 11, 1939. BRANDT 2,165,781

- LOUDSPEAKER SYSTEM Filed June 16, 1936 INVENTOR ERICH BRANDT ATTORNEY Pat entedJuly 11, 1939 UNITED STATES PATENT OFFICE,

LOUDSPEAKER SYSTEM Erich Brandt, Berlin-Lichterfelde, Germany, asslgnor to Telefu'nken Gesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, a

, corporation of Germany Application June 16, 1936, Serial No. 85,508' In Germany August 19, 1935 3 Claims. (01. 119-81) equipped with loudspeakers care must be taken I so that between loudspeaker and microphone, ,or between transmitter and receiver, there will arise no feedbacks either acoustic or electric in nature. For this reason it has been suggested to interpose between the sending branch and the receiving branch stopper means of the kind of feedback stoppers known from four-wire operation and which serve the purpose always to block 20 one branch so that any tendency toreaction between the two branches will be killed. Circuit organizations of this kind involve the drawback that they may occasionally be operated inadvertently and that mutual locking of both branches 25 may be occasioned with the result that satisfactory communication will be entirely precluded.

For this reason it has been suggested to provide, in lieu of such mechanical stopper arrangements, a scheme in which acoustic feedback 30 is avoided by that transmitter and receiver consist of two exactly equal systems or structures, say, two loudspeakers. The basic idea of this scheme is that by the use of two exactly alike systems there is a guarantee for the two halves 35 of the electrical shunt being of like impedance so that, at any rate, the electrical part of the coupling will not cause any difficulties at all. However, in experiments made with arrangements of this kind it has been discovered that instances will arise again and again where whistling in the amplifiers is produced in a scheme of the said sort, Further observations have shown that conditions of this nature are caused by certain local circumstances, say, owing to the fact 45 that persons who happen tobe inside the room where such a subscriber's station with loudspeaker is provided are at diiferent proximity to the various loudspeakers.

Now, the present invention is predicated upon the proper appreciation of this fact that the reasons underlying the coming out of step of such an arrangement may be sought in that the impedance of the two loudspeakers is not an exactly defined magnitude, but that it is also and I at the same time partly dependent upon the prevailing radiation resistance. Hence, as soon as the radiation resistance of the two loudspeakers, eitherwhen mounting or operating them, becomes unequal, balance or adaptation in the fork adjacent the apparatus is disturbed with 5 the result the feedback currents are set up, which, as soon as the amplification has reached a certain limit or is exceeded, are liable to lead to whistling or squealing of the equipment. Hence,

for this reason difllculties will always arise whenever, as a result of special circumstances, also the line-end simulation assumes unfavorable values.

Now, according to the present invention the said difficulties are obviated by that loudspeakers areemployed in which, by means of certain construction features, conditions are made so that the radiation thereof is made practically independent of changes inside the room and that the radiation resistance is thus made always the same in both of the two loudspeakers used. This end may be attained by that the active surface I through which the sound waves enter into the loudspeaker, are formed annular. In this case any alteration of the radiation resistance in one 5 direction, inside the room, means practically no change inthe total radiation resistance, inasmuch as the radiation resistance of the loudspeakers is not directive so that a variation in one sense would mean only a slight percentage value in the aggregate radiation resistance. For the identical reason, also the radiation resistance for the two loudspeakers that are used are practically equal inside the room seeing that the entire space is governing for both. To be sure, it is necessary in this connection that the two loudspeakers should be disposed eo-axially and that their position in the room should be practically alike.

In the attached drawing is shown an exempli- 4o fled arrangement which is intended to explain the invention in detail. Fig. 1 is a sectional view of the arrangement, Fig. 2 is a top view, and Fig. 3 is a circuit diagram illustrating the arrangement. The loudspeaker systems L1 and 5 L2 are co-axi'ally disposed inside a joint casing 6. The supply of the sound-waves is through the ducts 3 and 4 which have annular inlet surfaces.

It is merely broken at points 4 by partitions 5. The construction of the loudspeaker systems as well as of the casing, as to the rest, may be chosen at will to conform to rules and conventions prevalent in the building of loudspeakers. What is only essential in connection with the invention is the disposition of the system in space and the formation of the active surface for the en of the sound-waves.

Fig. 3 is a circuit diagram showing the connection of the two loudspeakers L1 and-La. U1 and U: are the two forked repeaters. An amplifier is provided foreach direction of operation or conversation. N is the simulation or network provided at the first fork. By means of this network the first fork U1 is perfectly balanced. Now, in-'- asmu'ch as evidently also the second fork U1 must be balanced if self-excitation of the system is to be precluded, it is necessary that the two loudspeakers Li and La should be exactly alike. This the loudspeakers act as microphones, are able to bring action upon both loudspeakers from all sides in a uniform and simultaneous way. Moreover, their radiation resistance will not be appreciably interfered with or aifected by persons who are present in the room seeing that they occupy only a very small fraction of the room or space served or covered by the loudspeakers. In other words,

the constancy or stability of the radiation resistance is governed by the construction of the two loudspeakers according to this invention. Moreover, a change in the radiation conditions would afl'ect both loudspeakers in a similar way seeing a that the circular inlet and outlet openings of the two loudspeakers are directly superposed, with the result that it is normally not even feasible to vary the radiation resistance of one of the loudspeakers without simultaneously and incidently affecting that of the other to the same degree, 4

-What I claim is: r I Y 1. Loudspeaker arrangement for subscribers stations free from feedba'cks, in which transmitter and receiver are disposed in a common enclosure and are electrically inter-connected by means of a fork or split connection, and consist of two substantially similar loudspeakers which are mounted axially, with the characteristic-features that the loudspeakers are so designed and disposed that their radiation resistance inside a given room will be practically alike, and that the active surface for the entrance of sound I waves is made annular.

2. In combination, a pair-oi loudspeakers disposed within a common enclosure, said speakers being coaxially mounted and'facing one another as regard to the direction of the emitted sound,

said enclosure having sound ports, ,one on'either sideof the speaker axis, and means within the enclosure for deflecting the sound radiation from each speaker through both of said sound ports.

3. The invention according to claim 2 wherein one of the loudspeakers is employed as' a sound transmitter and the other as a sound receiver, and both are substantially similar electrically whereby their radiation resistances are substantially the same.

ERICH BRANDT. 

